This invention relates to electrochemical cells and more particularly is concerned with the prevention of hydrogen gas pressure buildup in sealed galvanic cells.
It has long been an object of the battery industry to produce sealed cells in which the buildup of excessive gas pressure is avoided. This goal has been partially achieved in nickel-cadmium cells through the operation of such cells on the oxygen overcharge principle. The electrodes employed in this system are so designed that only oxygen is evolved on overcharge. The oxygen thus produced oxidizes the metal anode, thereby effectively preventing excessive gas pressure within the cell.
While the oxygen overcharge system is advantageous in that it makes possible a sealed battery, there are several inherent problems which limit its applications. For example, the anode must be made of a material which reacts rapidly with oxygen and the cathode material is limited to compounds which do not deteriorate on repeated overcharging and which do not form soluble materials on overcharge. In addition, the applications of this system are limited to rechargeable cell systems employing expensive components and there is no efficient way of recombining the hydrogen gas generated by corrosion in the normal course of battery use.
It has been proposed, to remove the hydrogen gas evolved during cell discharge, to utilize the reaction of hydrogen and oxygen to form water. In some battery systems, for example, a third electrode has been inserted to combine the hydrogen and oxygen produced during discharge. In the lead-acid battery system, palladium or platinum metal has been used to catalyze the reaction of hydrogen and oxygen to form water. Neither of these solutions has been completely satisfactory since the amounts of hydrogen and oxygen gas formed are not necessarily the stoichiometric amounts which will react to form water without leaving unreacted gas in the system. If there is unreacted hydrogen gas in the system, the internal pressure in sealed cells may increase to dangerously high levels or may lead to seal rupture.